We know that the environment will never be completely free of man-made toxic chemicals. So we naturally ask whether a very small level of a particular toxic is low enough to cause no harm. All municipal water is tested and regulated according to best estimates of Safe Dose. The EPA sets water standards, called “Maximum Contaminant Levels,” which are defined as the highest level of a contaminant that is allowed in drinking water. For example, for arsenic the MCL is set at 10 parts per billion.
It is worth considering this important and contentious issue. Is there is some small dose of a chemical, known to be damaging at moderate levels, below which no harmful effect will be produced?
It has long been assumed that there exists some Safe Dose for a toxic substance, such that if less than that amount is taken in by a person, there will be no detrimental effect. However, it has been found that levels initially thought to provide safety are often found to damage health when the agents are studied over longer periods of time.
Calomel, a compound of mercury, was used extensively as a cathartic (purgative) since the 16th century, so I presume that in small quantities it is somewhat safe. However, its use was later banned because repeated use, or use of too large a quantity, produces deadly symptoms of mercury poisoning. Present wisdom is that there is no safe lower dose for mercury or lead compounds because they accumulate in the body and, at very low concentrations interfere with normal functioning of the affected tissue. Many pesticides, DDT for example, accumulate in the body of animals, and become concentrated when that animal is eaten by another higher up on the food chain.
To determine the highest dose that fails to produce any observable ill effects in laboratory animals, toxicologists resort to using much higher doses so that ill effects show up in a few weeks or months. The technical challenge is to relate such high-dose laboratory conditions to real world low-dose conditions. If the severity of a health effect in test animals is measured for different dosages of a toxic substance, and the effects graphed against dose, the curve can be extrapolated (i.e., drawn back) to zero observed effect and the corresponding calculated dose is often assumed to be a Safe Dose that produces no long term effect on the health of the animals. But studies have shown that the extrapolation to zero-effect procedure is invalid for endocrine disrupters.
Scientists from six research institutions investigated the issue of dose and effect, specifically whether there is a threshold dose, i.e., a Safe Dose, and they investigated the effects of exposure to multiple toxics. They state that thresholds are plausible for acute toxicity (i.e., where the effect is rapid and evident), but it is uncertain whether this assumption is justified for chronic effects (i.e., where the effect is slow to manifest itself). Exposure levels that could be seen as sufficiently low when based on single agents, may actually not be safe in the context of the many other carcinogenic and mutagenic exposures. The bodies of most persons are contaminated with a hundred or more toxic chemicals.
Researchers studying the health of salmon reported on the devastating effects of the big volume agricultural organophosphate pesticide chlorpyrifos. They tested the effects of mixtures of chlorpyrifos and four other pesticides, and found a synergetic punch with the pesticides diazinon and malathion, which, in the presence of chlorpyrifos, killed all the exposed juvenile salmon, even when all concentrations were far lower that that at which they have an effect by themselves.
As experience with new toxic substances grow, and damaging effects start to show up for seemingly low dosage exposures, regulatory agencies have revised downward the Safe Dose. For Bisphenol-A the original Safe Dose was 5 mg/kg (body weight)/day; as effects on reproductive organs and development of animals became apparent, the Safe Dose was lowered to 0.10 mg/kg/day, and then to 0.0024 mg/kg/day. For the herbicide Atrazine, EPA’s original Safe Dose” in drinking water was 3 ppb; when is was discovered that frogs developed both male and female sex organs at lower levels, the Safe Dose was dropped to 0.1 ppb.
For induction of cancer the standard assumption is that there is no threshold. In essence, it is believed that every dose of one or more carcinogenic substances, no matter how small, probably induces cancer cells. A person can reasonably conclude that he or she should try hard to minimize the amount of toxic substances that enter their bodies.